The use of wireless communication is becoming more widespread. As the number of mobile units in the system increases, techniques are often used to improve communication. The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present.
In a typical wireless communication system, the mobile units used for communication typically include transceiver capable of transmitting and receiving messages. In most transceivers, the absolute values of the gain of transmit and receive paths are not precisely known without some form of calibration. In wireless systems that require well-controlled output power levels, some form of closed-loop power control scheme can be employed.
To calibrate a mobile unit in satellite-based CDMA communications systems, the mobile unit transceiver measures the power level of a signal received via a satellite repeater. Using this power measurement, along with knowledge of the satellite transponder downlink, the transmit power level, and the sensitivity of the mobile unit receiver, the mobile unit transceiver can estimate the path loss of the channel between the mobile unit and the satellite. The mobile unit transceiver then determines the appropriate transmitter power to be used for signal transmissions between the mobile unit and the satellite, taking into account the path loss measurement, the transmitted data rate, and the satellite receiver sensitivity.
The signals transmitted by the mobile unit to the satellite are relayed by the satellite to a Hub control system earth station. The Hub measures the received signal power from signals transmitted by each active mobile unit transceiver. The Hub then determines the deviation in the received power level from that which is necessary to maintain the desired communications. Preferably the desired power level is a minimum power level necessary to maintain quality communications so as to result in a reduction in system interference. After determining the deviation in received power, the Hub transmits a power control command signal to each mobile user so as to adjust or "fine tune" the transmit power of the mobile unit. This command signal is used by the mobile unit to change the transmit power level closer to a minimum level required to maintain the desired communications. As channel conditions change, typically due to motion of the mobile unit, both the mobile unit receiver power measurement and the power control feedback from the Hub continually readjust the transmit power level so as to maintain a proper power level. The power control feedback from the Hub is generally quite slow due to round trip delays through the satellite requiring approximately 1/2 of a second of propagation time.
In contrast to the satellite-based wireless system, in a terrestrial-based wireless system, calibration must take into account the fact that the distance between the mobile units and the cell sites can vary considerably. For example, one mobile unit may be located at a distance of five miles from the cell site while another mobile unit may be located only a few feet away. The variation in distance may exceed a factor of one hundred to one. The terrestrial channel in such a wireless system experiences a propagation loss characteristic (as did the satellite channel), but in the terrestrial channel the propagation loss characteristic corresponds to an inverse fourth-power law, i.e., the path loss is proportional to the inverse of the path distance raised to the fourth power. Accordingly, path loss variations may be encountered which are on the order of over 80 dB in a cell having a radius of five miles.
U.S. Pat. No. 5,257,283, to Gilhousen et al. proposed one method of calibrating mobile units in a terrestrial-based wireless system. Specifically, Gilhousen discloses a closed-loop power control scheme in which the base station indicates to the transmitter that the transmitter is transmitting at too high a power level or too low a power level based on signals the base station receives. The transmitter is then responsible for adjusting its power level to conform to the demands of the base station. One problem with this approach is that it requires a central controller to make a determination as to whether the power level is too high or low.
If the transmitter gain, receiver gain, and path loss are known, each transmitter can adjust its output power level to the determined power level without employing closed-loop control. The transmitter gain and receiver gain can potentially be calibrated off-line. However, such off-line calibration is sensitive to device aging and temperature drift.
What is needed is a calibration methodology that overcomes deficiencies in the prior art.